This invention relates generally to electronic systems and, more particularly, to electronic circuits, including integrated circuits, that require current sensing. In addition, the present invention relates to integrated circuit fabrication and, in particular, to a method for sensing the current in a field effect transistor (FET) device. Generally, FET devices can be classified as either junction field effect transistor (JFET) devices or metal oxide semiconductor field effect transistor (MOSFET) devices. The present invention applies to both JFET and MOSFET devices, as well as to hybrid circuits, assembled board circuits, and any other type of electrical circuit in which sensing of current in a FET device is performed. Circuits which require current sensing include voltage and current sources, voltage and current references, and various regulator circuits, among others.
Many prior art circuits employ means to control and limit the current flowing through their terminals, thus protecting the circuit itself, the load, or both, from the effects of, and possible damage resulting from, excessive current. Voltage sources and voltage regulators employing current limit protection are one example of such circuits. With the increasing use of smaller power supply voltages and battery operated systems and the advent of low dropout voltage regulators, the need to sense current in the power device, while minimizing peturbations to the system, becomes more important.
The most common way to sense current is to monitor the voltage drop across a current sense resistor, inserted in the current path. In accordance with Ohm""s law, the voltage drop across the current sense resistor is directly proportional to the current flowing through that resistor. The current sense resistor may be replaced by any device or circuit with substantially resistive characteristics. However, this approach is disadvantageous in several respects. First, all of the output current, possibly large, flows through the sense device or circuit; resulting in undesired power loss and/or heat dissipation. Thus, the sense device or circuit must be capable of dissipating a large amount of power/heat, which increases its cost and size. Attempts to minimize power loss by minimizing the resistance of the sense device or circuit encountered difficulties in implementing and controlling those small resistances. In addition, the voltage drop across the sense device or circuit which is inserted in series with the load current path, is unacceptable in some applications, such as in low dropout (LDO) voltage regulators, for instance, and it is generally undesirable in any application.
The prior art is replete with attempted solutions to the current sensing problems discussed above. U.S. Pat. No. 4,021,701 teaches sensing the current in a bipolar transistor by means of a scaled down transistor, whose base and emitter are connected in parallel with the base and emitter terminals of the first transistor. The transistor whose current is evaluated is often called the power transistor. This scaled down transistor is often called the sense transistor because it is used to sense the current in the power transistor. This patent teaches that the power transistor and the sense transistor must be of the same type; that is, they must both be either npn transistors or pnp transistors. The power transistor and the sense transistor have common base terminals and common emitter terminals, so the emitter current flowing in the sense transistor is substantially proportional to the emitter current flowing in the power transistor. The proportionality factor depends, essentially, on the form factor ratio between the sense and power transistors. As a first approximation, this is the ratio of the emitter area of the sense transistor to the emitter area of the power transistor. The collector current in the sense transistor is approximately equal to its emitter current. If a resistor is connected in series with the collector of the sense transistor, a current that is essentially proportional to the current flowing in the power transistor will flow through it. Thus, the current that flows in the power device can be monitored, by observing the voltage drop across said resistor.
U.S. Pat. No. 5,272,392 teaches current sensing in the case of a MOSFET, by means of a smaller, scaled down MOSFET, which is referred to as the sense transistor or sense MOSFET. The sense transistor is of the same type, NMOS or PMOS, as the power MOSFET. The source and gate terminals of the sense transistor are connected to the source and gate terminals of the power transistor, respectively. As illustrated in FIG. 1, a resistor is connected between the drain terminal of the sense MOSFET and the drain terminal of the power MOSFET. Thus, a smaller current, substantially proportional to the current flowing in the power MOSFET, flows through the drain terminal of the sense MOSFET and hence through the resistor. The proportionality factor depends, essentially, on the scale factor between the sense MOSFET and the power MOSFET. The scale factor depends, as a first approximation, on the W/L ratios of the two transistors (FET channel width over channel length). According to Ohm""s law, the resistor develops a voltage drop across itself, proportional to the current flowing through it. This current is, in turn, substantially proportional to the current flowing in the power MOSFET. The voltage drop across the resistor may thus be used as a measure of the current flowing in the power MOSFET, multiplied by a constant. At the same time, the voltage drop across the resistor represents the difference between the drain voltage of the power FET and the drain voltage of the sense FET. This causes the sense FET and the power FET to operate at different drain-to-source voltages, thus introducing an error in the current mirror effect. This error becomes more and more important, as the drain-to-source voltage becomes smaller.
Yet another approach to current sensing is disclosed in U.S. Pat. No. 5,867,015. This patent teaches current sensing in a MOSFET, which is herein referred to as the power MOSFET, by connecting in parallel the source and gate terminals, respectively, of a power MOSFET and of a smaller, scaled down MOSFET, which is herein referred to as the sense transistor or sense MOSFET. This prior art solution to current sensing, a circuit illustrated in the example of FIG. 2, eliminates the need for a sense resistor connected in series with the drain terminal of the sense MOSFET. Instead, the sense resistor is replaced with a circuit known as a current conveyor, consisting of components Q1, Q2, M3, M4. The first terminal, A, of the current conveyor circuit is connected to the drain terminal of the sense MOSFET M2. The second terminal, B, of the current conveyor circuit is connected to the drain terminal of the power MOSFET M1. The third terminal, C, of the current conveyor circuit is connected to ground, in the case of a PMOS type power transistor, or to the upper supply voltage rail, in the case of an NMOS type power transistor. The current conveyor circuit is used to transfer the voltage at the drain terminal of the power MOSFET to the drain terminal of the sense MOSFET. At the same time, it causes a current, equal to the current flowing in the drain terminal of the sense MOSFET, to flow in the other terminal, B, of the current conveyor circuit, thus forcing that current to flow into the drain terminal of the power MOSFET. The current that flows in the sense MOSFET is, generally, much smaller than the current flowing in the power MOSFET by a factor which is determined, essentially, by the scale ratio between the sense MOSFET and the power MOSFET. As a first approximation, the scale ratio is given by the W/L ratios of these two devices, where W is the channel width and L is the channel length. Because this factor is usually very large, the supplementary loading of the power MOSFET drain terminal with a much smaller current, is of no practical effect. As disclosed in this patent, the current conveyor circuit comprises two bipolar transistors and a MOSFET current mirror, which is referred to ground. Using the reference side of this current mirror (device M3 in FIG. 2), another current, substantially proportional to the current flowing in the power MOSFET, can be derived, by using another current mirror circuit, consisting of transistors M3 and M5 in FIG. 2. The current thus obtained can be used to monitor the power MOSFET current and may be scaled up or down, as needed. The advantage of the circuit described in this prior art reference, over the previous one, is that the drain voltage is essentially the same for both the power FET and the sense FET. The voltage at node A equals the voltage at the base of transistor Q2, plus the base-emitter voltage of transistor Q2. The voltage at the base of transistor Q2 equals the voltage at node B, minus the base-emitter voltage of transistor Q1. Transistors Q1 and Q2 have essentially the same collector current, due to the current mirror M3-M4. This makes their base-emitter voltages equal and leads to equal voltages at nodes A and B, in FIG. 2. Hence, the power FET and the sense FET operate at essentially equal drain-to-source voltages. This improves M1-M2 current mirror performance, which is of increasing importance, as the operating drain-to-source voltage becomes smaller.
The two bipolar transistors included in the current conveyor circuit of U.S. Pat. No. 5,867,015 must be fully floating. That is, they must have all terminals uncommitted. Because the vast majority of CMOS processes do not offer bipolar transistors with uncommitted terminals, and because simply replacing the bipolar devices in the current conveyor circuit disclosed in this reference with appropriate MOSFET devices leads to non-functional circuitry, a novel solution is proposed in accordance with the present invention.
Generally, and in accordance with one embodiment of the present invention, a circuit for parallel sensing of the current in a FET, referred to as a power FET, includes a sense FET and a current conveyor circuit employing exclusively FET devices. All FET devices may be MOSFET devices or JFET devices. The sense FET and the power FET have their gate terminals connected together and their source terminals connected together. This allows the implementation of the circuits of the present invention in integrated circuits realized in CMOS processes, without the need for bipolar devices. The circuits of the present invention may also be employed in situations where JFET devices are provided in the process, as well as in discrete component implementations.